Method of producing microstructured metal sheets

ABSTRACT

In a method of producing microstructured metal sheets, microstructures are made in a metal sheet surface in a continuous embossing step on one side of the metal sheet, while the surface structure of the other side is essentially not changed. The embossing step is carried out by means of an embossing roller which has a negative of the microstructure to be embossed. The flanks of the microstructure form an angle of at least 5° with respect to the perpendicular line, and the negative contains compensating structures in the regions surrounding the microstructure for rendering the forming uniform along the roller slit.

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] This application claims the priority of German Patent Document No. 101 08 469.2-14, filed Feb. 22, 2001, the disclosure of which is expressly incorporated by reference herein.

[0002] The invention relates to a method of producing microstructured metal sheets as well as to the use of the method.

[0003] German Patent Document DE 198 01 374 A1 discloses methods which are suitable for producing microstructured metal sheets, such as etching, milling, embossing, punching or wire-EDM.

[0004] In the prior art, microstructured metal sheets have been produced by galvanic methods (removal or build-up of material), chemical methods (etching), electrical methods (eroding), electrochemical methods (ECM), mechanical methods (HSC milling, engraving) or laser removal methods. The disadvantage of all above-mentioned methods is the high production costs. The last two of the above-mentioned methods have the disadvantage of a high carrying-in of heat which warps the metal sheets. In the case of the galvanic, chemical and electrochemical methods, costly disposal or recycling of the used materials is necessary. Furthermore, at the conclusion of the respective process, another process step, finish rolling, has to be added in order to obtain highly parallel metal sheets, which are required for the further course of the production. In addition, in the case of mechanical methods (etching), for reasons of corrosion, it is not possible to use just any material for the production of metal sheets. High-alloy Cr—Ni steel and Ni base alloys respectively are not suitable for etching and more expensive materials must therefore be used.

[0005] It is therefore an object of the invention to provide a cost-effective method which is suitable for mass production and has a high dimensional accuracy of the metal sheets while the influence of heat during the embossing is only minimal.

[0006] To achieve this object, the present invention provides a method of producing microstructured metal sheets, which is described hereinafter.

[0007] In the method according to the invention, a very good surface quality can be attained in a particularly advantageous manner, which surface quality does not promote the corrosion caused by aggressive media. In the case of etching, on the other hand, the grain boundaries are exposed and promote corrosion. Furthermore, the method is not harmful to the environment.

[0008] Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1a is a schematic view of the embossing of a metal sheet by means of a structured embossing roller; and

[0010]FIG. 1b is a cutout of FIG. 1a;

[0011]FIG. 2a illustrates a compensating structure that has ribs and/or grooves having the criss-cross configuration;

[0012]FIG. 2b illustrates a compensating structure that has a mesa-like configuration;

[0013]FIG. 3a is a schematic view of a possible arrangement of a negative structure on an embossing roller which, on the one hand, contains the microstructure and, on the other hand, contains the compensating structures; and

[0014]FIG. 3b is a cutout of FIG. 3a.

DETAILED DESCRIPTION OF THE DRAWINGS

[0015] A method according to the invention is directed to the microstructuring of metal sheets. Microstructures are made in a continuous embossing step into the surface on one side of a metal sheet, while the surface of the other side is essentially not changed. The embossing step is carried out by means of an embossing roller 1 which has a negative of the microstructure 3 to be embossed. The flanks 5 of the microstructure form an angle of at least 5° with respect to the perpendicular line. The regions of the roller surrounding the microstructures have the negative of compensating structures 4 to make the embossed metal sheet uniform along the roller slit. This is schematically illustrated in FIGS. 1a and 1 b.

[0016] The microstructure 3 may have any suitable structures. For example, the microstructure may have grooves and/or ribs that are arranged in any suitable manner. In the illustrated embodiment, the microstructure 3 includes grooves and/or ribs that form a plurality of concentrically arranged rectangles. When embossed, the area of the metal sheet containing the microstructure 3 is plastified, and the grains of the metal material are crushed into finer grains due to the thinness of the metal sheet and the high degree of deformation.

[0017] The compensating structures 4 preferably are used to make the force or pressure applied to the metal sheet 2 during embossing substantially uniform in all areas of the metal sheet 2, so that the areas of the metal sheet 2 surrounding the microstructure 3 are also pressed and plastified to crush the grains of the metal material into finer grains. Without the compensating structures 4 the areas of the microstructures 3 would have relatively small grains, while the areas surrounding the microstructures 3 would have relatively large grains. The interface between the large and small grain areas may damage the metal sheet 2.

[0018] The compensating structure may have any suitable configurations. For example, the compensating structure may have ribs and/or grooves having the criss-cross configuration shown in FIG. 2a, or it may have the mesa-like configuration shown in FIG. 2b.

[0019] Preferably, the width of the patterns on the roller is greater than the width of the metal sheet to ensure that all areas of the metal sheet are embossed. In addition, the depth and width of the patterns of the compensating structure are similar to or the same as those of the patterns of the microstructure.

[0020] Metal sheets 2 suitable for use with the invention are metallic and may be made of, for example, steel, particularly stainless steel, such as Cr—Ni steel, and of aluminum, Ni, Ni alloys, Pd, Pd alloys, Cu, Cu alloys.

[0021] Before embossing, the metal sheet (2) can be heated by means of inductive heating or by means of infrared radiators, and the temperature may be in the range of room temperature to 300° C. and may depend on the material. During the subsequent embossing step, the so-called cold-rolling at room temperature to 300° C., the material is completely plastified. In other words, the grains of the metal material are smashed as a result of the narrow material thickness and the high degree of deformation. The roller pressure of the embossing step is in the range of approximately 30 to 1,000 tons, preferably in the range of from 50 to 500 tons. During embossing, a reduction of the material to be embossed takes place, and the material is also highly compacted in the roller slit. To reduce the possibility of the material becoming jammed when passing through the roller slit, the embossing (microstructure) 3 on the roller 1 has an angle of at least 5° at the flanks 5, and the negative in the regions surrounding the microstructure contains compensating structures 4 which render the forming uniform along the roller slit and along the metal sheet width respectively and therefore lead to uniform advancing. As a result, cracks in the metal sheet are advantageously avoided and a good surface evenness is ensured because the material flows uniformly forward along the width of the roller slit. FIG. 3a is a schematic illustration of, and FIG. 3b is a sectional view of, the roller with a uniform arrangement of the microstructures and the compensating structures 4. However, the high deformation necessarily leads to internal tensions in the metal sheet. Therefore, the embossing roller 1 itself may have a crowned shape.

[0022] With respect to the embossing step, the method can be implemented in a force-controlled and/or path-controlled manner. In the case of a force-controlled implementation of the method, the roller force is kept constant by way of a contact and control mechanism. The roller slit height can be variably adjusted as a function of the local degree of forming and the input cross-section of the metal sheet. In contrast, in the case of a path-controlled implementation of the method, the roller slit height is kept constant by way of a contact and control mechanism, whereas the roller force is variably adjustable as a function of the local degree of forming and the input cross-section of the metal sheet. The method may also be a combination of a force-controlled and path-controlled implementation. During embossing, a new grain formation (recrystallization) takes place which is comparable to the old grain structure.

[0023] After the embossing step, the metal sheet is subjected to a calibrating step. This calibrating pass is used for equalizing the embossing profile. In the calibrating step, embossed metal sheet 2 is treated with a plane roller to adjust the thickness and dimension of the embossed metal sheet.

[0024] After the calibrating step, die cutting may be performed and the metal sheet 2 may be cut into designed sizes. Each cut metal sheet preferably contains the microstructure 3 and may also contain the compensating structures 4.

[0025] To reduce internal tensions of the microstructured metal sheet, the calibrating step is followed by at least one annealing step. The temperatures during the annealing process are in the range of from approximately 500 to 1,200° C. depending on the type of the material to be embossed. The annealing process is carried out with the exclusion of oxygen, preferably in a hydrogen-containing and/or nitrogen-containing atmosphere, for reducing or preventing an oxide skin formation on the surface of the material.

[0026] Subsequently, after the calibrating and/or annealing step, the microstructured metal sheet is subjected to a straightening process at room temperature. The continuous process can be uncoupled between the embossing and the calibrating step, and the process steps occurring after the embossing step can be continued in a separate system.

[0027] The microstructures produced according to the method of the invention have a depth of approximately from 0.1 to 0.5 mm, the metal sheet experiencing a thinning by approximately 50% during the process.

[0028] The method is preferably used for producing a microreactor, a micro heat exchanger, an oil-heated, catalytically or hot-gas heated evaporator, a membrane module for the separation of H₂ or a fuel cell, particularly a polymer electrolyte membrane fuel cell, in which case the devices produced according to the method of the invention are equally suitable for hydrogen fuel cells, reformate-operated fuel cells or direct methanol fuel cells.

[0029] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 

What is claimed:
 1. Method of producing microstructured metal sheets, comprising: making microstructures in a metal sheet surface in a continuous embossing step on one side of the metal sheet, while a surface on the other side is essentially not changed, carrying out the embossing step by means of an embossing roller which has a negative of the microstructure to be embossed, wherein flanks of the microstructure form an angle of at least 5° with respect to the perpendicular line, and the negative contains compensating structures in regions surrounding the microstructures for rendering forming uniform along a roller slit.
 2. Method according to claim 1, further comprising heating the metal sheet before the embossing step.
 3. Method according to claim 1, further comprising carrying out the embossing step in at least of a force-controlled and path-controlled manner.
 4. Method according to claim 1, wherein roller pressure of the embossing step is in a range of approximately 30 to 1,000 tons.
 5. Method according to claim 1, wherein, after the embossing step, the metal sheet is subjected to a calibrating step.
 6. Method according to claim 5, wherein the calibrating step is followed by at least one annealing step for reducing internal tensions of the microstructured metal sheet.
 7. Method according to claim 6, wherein the annealing process is carried out with the exclusion of oxygen.
 8. Method according to claim 5, wherein, after at least one of the calibrating and annealing steps, the microstructured metal sheet is subjected to a straightening at room temperature.
 9. Method according to claim 1, wherein the microstructures preferably have a depth of from approximately 0.1 to 0.5 mm.
 10. Method according to claim 1, further comprising thinning the metal sheet by approximately 50%.
 11. Use of the method according to claim 1, preferably for the production of a microreactor, a micro heat exchanger, an oil-heated, catalytically or hot-gas heated evaporator, of a membrane module for the separation of H₂ or of a fuel cell.
 12. A method of producing a microstructured metal sheet, comprising: embossing a microstructure and compensating structures surrounding the microstructure on a surface on only one side of a metal sheet, wherein flanks of the microstructure form an angle of at least 5° with respect to the perpendicular line.
 13. The method according to claim 12, further comprising heating the metal sheet before embossing.
 14. The method according to claim 12, further comprising carrying out the embossing step in at least one of a force-controlled manner and a path-controlled manner.
 15. The method according to claim 12, comprising applying a force in the range of approximately 30 to 1,000 tons to the surface of the metal sheet in the embossing step.
 16. The method according to claim 12, comprising applying a force in the range of approximately 50 to 500 tons to the surface of the metal sheet in the embossing step.
 17. The method according to claim 12, comprising calibrating the metal sheet after the embossing step.
 18. The method according to claim 17, comprising cutting the metal sheet in sheets of desired sizes.
 19. The method according to claim 17, comprising annealing the microstructured metal sheet to reduce internal tensions after the calibrating step.
 20. The method according to claim 19, comprising annealing the microstructured metal sheet with the exclusion of oxygen.
 21. The method according to claim 19, comprising straightening the microstructured metal sheet at room temperature after at least one of the calibrating and annealing steps.
 22. The method according to claim 12, wherein the microstructures preferably have a depth of from approximately 0.1 to 0.5 mm.
 23. The method according to claim 12, further comprising thinning the metal sheet by approximately 50%.
 24. The method according to claim 12, wherein the compensating structure has at least one of ribs and grooves having a criss-cross configuration.
 25. The method according to claim 12, wherein the compensating structure has a mesa-like configuration. 